The chemical and physical properties of silicon carbide make it an excellent material for high temperature structural applications. These properties include good oxidation resistance and corrosion behavior, good heat transfer coefficients, low expansion coefficient, high thermal shock resistance and high strength at elevated temperature. It is in particular desirable to produce silicon carbide bodies having high density and suitable for engineering material uses, such as for example high temperature gas turbine applications. Silicon carbide is a preferred material for such use, because it can withstand stresses at higher temperatures than conventional materials, and can therefore lead to greater efficiency in the transformation of energy.
Methods of producing high density silicon carbide bodies have heretofore included reaction bonding (also known as reaction sintering), chemical vapor deposition and hot pressing. Reaction sintering involves the use of silicon impregnants to upgrade the density of the silicon carbide and is useful for many applications, but is undesirable where excess silicon exuding from the silicon carbide body would be detrimental. Silicon carbide deposition is impractical for producing complex shapes, and hot pressing (the production of high density silicon carbide bodies by simultaneous application of heat and pressure) is impractical for some shapes, since the pressure required during the hot pressing operation deforms the silicon carbide body and requires that only relatively simple shapes can be produced by this method.
According to applications Ser. Nos. 584,226 and 790,354, there was provided a sintered ceramic body having a high proportion of silicon carbide and a high (greater than 75% theoretical) density, and a process and raw batch for the production of such ceramic bodies, which did not require the use of finely divided "beta" (cubic crystal structure) silicon carbide.
Production of pressureless sintered silicon carbide, and hot pressed silicon carbide has been the subject of substantial inventive effort in recent years. In addition to the patent and applications referred to under "Cross References to Related Applications," all of which are assigned to The Carborundum Company, reference is made to G. Q. Weaver et al, U.S. Pat. No. 3,836,673, patented Sept. 17, 1974 and G. Q. Weaver, U.S. Pat. No. 3,998,646, patented Dec. 21, 1976, both assigned to Norton Company; as well as Svante Prochazka, U.S. Pat. Nos. 3,852,099, patented Dec. 3, 1974; 3,853,566, patented Dec. 10, 1974; 3,954,483, patented May 4, 1976; 3,960,577, patented June 1, 1976; 3,968,194, patented July 6, 1976; 3,993,602, patented Nov. 23, 1976; 4,004,934, patented Jan. 25, 1977; 4,023,975, patented May 17, 1977; and 4,041,177, patented Aug. 9, 1977; and Johnson et al, U.S. Pat. No. 4,031,178, patented June 21, 1977, all assigned to General Electric Company.
In none of the above-identified patents of General Electric Company and Norton Company is there disclosed pressureless sintered alpha silicon carbide ceramic bodies having equiaxed microstructures. The disclosure of a process which comes the closest to this objective is probably contained in U.S. Pat. No. 4,041,117, wherein there is disclosed a process comprising providing a substantially homogeneous particulate dispersion or mixture, wherein the particles are sub-micron in size, of beta silicon carbide powder, alpha silicon carbide seeding powder, boron additive and a carbonaceous additive which is free carbon or a carbonaceous organic material which is heat-decomposable to produce free carbon, shaping the mixture into a green body, and sintering the green body at temperatures ranging from about 1950.degree. C. to 2300.degree. C. in an atmosphere in which the green body and resulting sintered body is substantially inert, to produce a sintered body having a density of at least 80% of the theoretical density for silicon carbide and a substantially uniform, relatively fine-grained microstructure wherein at least 70% by weight of the silicon carbide present is composed of alpha silicon carbide in the form of platelets or elongated grains which may range in the long dimension from about 5 to 150 microns, and preferably from about 5 to 25 microns.
For some applications of sintered silicon carbide bodies, however, there are advantages to employing a sintered silicon carbide body having an equiaxed microstructure, as opposed to a structure in the form of platelets or elongated grains. Other factors being equal, there are differences in mechanical properties, primarily strength, which depend upon the largest flaw present in a particular sintered ceramic body. The large grains in the form of platelets or elongated grains act as large flaws, and accordingly there is an inverse correlation between the strength of a sintered ceramic body and the largest grain size observable in the microstructure. In other words, a fine-grained equiaxed microstructure is inherently stronger and possessed of other more desirable mechanical properties than a material which is otherwise the same, but has larger grains.